Many attempts are made to extend or prolong the chain of an epoxy resin for the production of cationic resins, particularly for cationic resins used as binders for cationic electrodeposition (CED) in order to favorably influence the adhesion and flexibility of crosslinked films without adversely influencing the corrosion resistance. Thus, attempts to extend or prolong the chain length of epoxy resins with diphenols leading to an increase in molecular size of low molecular epoxy resins are described in U.S. Pat. Nos. 4,339,368 and 4,339,369, and in DE-OSS Nos. 23 39 398 or 25 31 960. According to the teachings of those patents, although an increase in molecular size is realized, no substantial influence on the adhesion or film-forming temperature is attained. As disclosed in DE-AS No. 19 30 949, the adhesion and the flexibility of a film can be imporved through partial esterification of epoxy resins with long chain monocarboxylic acids and/or dipolycarboxylic acids, such as the maleinized oils or maleinized polybutadienes, acidic alkyd resins, or copolymers. Further, according to U.S. Pat. Nos. 4,104,147 or 4,148,772, chain prolongation can be effected with cationic modified epoxy resins through reaction of such resins with polyols with at least two primary hydroxy groups, whereby the rupture voltage, the film-forming temperature, and the film flexibility, respectively, can be improved. Recently it has been established that the aforesaid modifications are not sufficient to meet the increasingly stringent requirements of coatings, particularly for use by the automobile industry.
According to more recent disclosures, the adhesion of cathodically deposited films can be improved if the cationic binders employed carry an amide structure. However, the method of introducing the amide structure is of essential importance, since, for example, aminoamide compounds or maleinized fatty acids and polyamines, such as are described in U.S. Pat. Nos. 4,274,989 or 4,036,795, show no improvement with respect to adhesion.
Another proposal for introducing amide and/or urea groups for the chain prolongation of an epoxy resin, effected through diamines carrying the urea or amide groups, is disclosed in EP-A No. 2-0 137 459. The basic groups responsible for the cationic character of the binders are introduced through additional amine functionally. In U.S. Pat. No. 4,563,515 carboxylic acid amide structures are introduced into an epoxy resin through special secondary amines which simultaneously convey to the molecule, either partially or totally, the cationic character necessary for water-dilutability. U.S. Pat. No. 4,568,709 uses aminoalkylated phenols for the preparation of self-crosslinking electrodeposition paint binders. More specifically, the patent discloses cathodically depositable electrodeposition paint binders comprising the reaction product of an epoxy resin with at least two epoxy groups with amines carrying primary amino groups and, optionally, with a carboxylic acid compound to provide an epoxy resin-amino adduct which is then reacted with a phenol and formaldehyde.